Environ. Sci. Technol. 1996, 30, 2318-2321
Energy-Dispersive X-ray Fluorescence Methods for Environmental Characterization of Soils STEVEN J. GOLDSTEIN,* ALICE K. SLEMMONS, AND HEATHER E. CANAVAN Chemical Science and Technology Division, MS K484, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
With recent requirements for rapid, field-based methods for environmental characterization, we have evaluated energy-dispersive X-ray fluorescence (EDXRF) techniques for elemental analyses of soils at Los Alamos using laboratory, transportable, and portable instruments. Fundamental parameters provide reasonably accurate standardization, and spectral interferences are generally absent. Detection limits are below screening action levels or background soil abundances for all elements of concern except As and Be. Results for certified materials indicate that accuracy is typically better than (10%, although some elements have few or no suitable reference materials to evaluate accuracy. Portable and fixedbase instruments typically give consistent results. However, large positive biases (2-78×) are generally found between EDXRF and standard EPA nitric acid digestion methods. This reflects the fact that EDXRF measures total amounts of the analyte, whereas EPA methods measure only the components labile in nitric acid and not the matrix. Consequently, EDXRF and EPA methods are not directly comparable for pristine soils, whereas contaminated soils should give more comparable results for the two techniques. Our data indicate that EDXRF can vastly exceed analytical requirements for field screening, and that this simple and fast technique can yield fully quantitative elemental analyses for soils in environmental studies.
Introduction Recently, there has been a growing need for rapid, fieldbased methods for elemental analyses of soils and other difficult matrices for environmental characterization and restoration purposes (1). Energy-dispersive X-ray fluorescence spectroscopy (EDXRF) has several potential advantages for this type of analysis (2-8). It is nondestructive * Author to whom all correspondence should be addressed; telephone: 505-665-4793; fax: 505-665-5982; e-mail address:
[email protected].
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with rapid throughput and simple sample preparation amenable to a field setting, a broad range of elements from Na to U are characterized simultaneously, and the sensitivity of ∼10 ppm is appropriate for field screening for most metals. Possible disadvantages are related to spectral interferences for certain elements such as Pb and As and poorer sensitivity for lighter elements, with hazardous constituents of atomic number lower than Na such as Be not detected. For soils and other complex matrices, empirical methods for calibration can be difficult or cumbersome, and theoretical calibration methods such as fundamental parameters models are not always viewed as reliable (e.g., ref 9). It can also be potentially difficult to directly compare total analyte measurements by EDXRF with the more standard acid leach methods designed to analyze only acid-labile components, although most studies have shown good agreement for contaminated soils (2, 3, 5, 8, 10). However, recent developments in field-based EDXRF have potentially overcome many of these problems (e.g., ref 4). High resolution Si(Li) detectors have improved energy resolution dramatically, thereby reducing spectral interferences. The development of personal computers with high speed and memory has also allowed fundamental parameter algorithms to be quickly performed using multiple standards, resulting in rapid and more accurate standardization and analyses for multicomponent, complex matrices over standard empirical methods (9, 11, 12). Here, we evaluate the EDXRF technique for elemental analyses of soils at Los Alamos with two fixed-base and field-transportable instruments and compare results with a portable instrument and standard nitric acid/ICP methods mandated by the Environmental Protection Agency (EPA). We show that fixed-base and field-transportable EDXRF techniques provide data of sufficient sensitivity, accuracy, and precision in almost all cases to meet the data quality objectives for environmental characterization purposes at Los Alamos.
Experimental Section Sample preparation and analysis follow techniques described by Watson et al. (6). Briefly, 20-30-g samples are dried and physically homogenized, first by drying in a microwave oven for 5 min or in a convection oven overnight. Samples with large objects are sieved through a 10-mesh polypropylene screen to remove the coarse fraction. Samples are then milled and mixed for 5 min in a zirconia ball mill assembly and are sieved through a 100-mesh nylon screen, from which a ∼10-g fraction passing through the screen (2 × DL) for the Laboratory EDXRF element
concn range
accuracy range
av accuracy
K Ca Ti Cr Mn Fe Ni Cu Zn Ba Pb Th U
1.00-3.49% 0.83-5.90% 900-8600 ppm 61-160 ppm 370-3950 ppm 2.37-6.00% 47-94 ppm 22-65 ppm 94-246 ppm 400-2000 ppm 28-130 ppm 17-990 ppm 19-650 ppm
92-141% 91-129% 0-95% 97-164% 81-133% 91-114% 69-170% 83-124% 84-128% 97-117% 95-122% 95-117% 103-137%
105% 104% 83% 118% 102% 100% 106% 96% 97% 104% 109% 109% 120%
biasa no no ? no no no no no no no no no no
a
Elements marked with ? may show a small bias based on the accuracy results.
and Cd, no check standards were available with high enough concentrations to permit quantification of these elements and an estimate of accuracy. Accuracy results for the transportable EDXRF were similar to those for the laboratory instrument. Results for the portable instrument (Spectrace 9000) and fixed-base instrument are compared in Table 4 for a set of 25 soil samples from Los Alamos. These results are obtained on the same sample cup for both instruments, hence differences reflect biases due to instrumental analysis only. Consistent results are obtained for the two instruments, with agreement to within 20% for most elements. However, some elements show large positive biases (factor of 5-40) for the portable instrument including As, Th, and U. This may be due to the somewhat poorer energy resolution and greater spectral overlap for these elements with the portable instrument, which has a HgI detector. Nominal energy resolution for the fixed-base and portable instruments are 160 and 240 eV for the Mn K-R, respectively. In contrast, large biases are found between EDXRF and standard nitric acid digestion/ICP methods prescribed by
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TABLE 5
Comparison of EDXRF and Nitric Acid Dissolution/ ICP Data element no. of samples av XRF/ICP standard deviation commentsa K Ba Ca Cu Cr Pb Fe Mn Zn As Ti
20 20 20 7 6 17 20 20 20 1 1
77.5 6.2 3.9 3.3 3.1 2.6 2.3 2.1 1.1 1.0 0.8
49.9 3.4 1.7 1.4 1.9 1.5 0.9 1.0 0.4 0.4
> > > > > > > > ) ? ?
a Elements marked with ) show no difference between methods, whereas elements with > indicate a significant difference. Elements with ? have too few quantifiable data to make a valid comparison. No comparison was possible for Cd, Hg, Sb, Se, Th, and U since results for these analytes were below detection.
EPA, as shown in Table 5. Average XRF/ICP results are 78 for K; 2-6 for Mn, Fe, Pb, Cr, Cu, Ca, and Ba; and 0.8-1.1 for As, Ti, and Zn. Although the two methods utilized distinct sample preparation methods, the XRF/ICP differences are too large and consistent to be due to sampling variability between the procedures. The divergent results most likely reflect the fact that the XRF technique is measuring total amounts of the analyte, whereas standard EPA methods are measuring only the nitric acid-digestible part of the soil and not the matrix. These results are somewhat consistent with prior studies comparing wet chemical analysis after hydrofluoric acid digestions with nitric acid digestions. The hydrofluoric acid digestion provides total dissolution of the sample, whereas the nitric acid digestion does not dissolve the silicate matrix constituents. Results of this comparison show that totally dissolved samples can have up to factors of 2-10 greater concentrations for certain elements than nitric aciddigested samples (15-17). From these data, it is evident that XRF and EPA methods are not directly comparable for
relatively pristine soils. For these samples, the EDXRF technique will provide a much more conservative estimate of elemental contamination than standard EPA procedures. However, in contaminated soils where most of the analyte is nitric acid-leachable, EDXRF and standard EPA methods should give more similar results. These data indicate that the EDXRF technique, when combined with steps to create a physically uniform and reproducible sample, vastly exceeds capabilities required for field screening of environmentally contaminated sites. In most types of environmental studies and others where accuracy of ∼(10% is required, this simple and fast technique can be used as a fully quantitative method for elemental analysis of soil samples.
Acknowledgments Ron Conrad and Albert Dye kindly provided the portable XRF data. Cathy Smith provided the comparison data for the EDXRF and standard EPA methods and also helped with constructive criticism. We also thank the anonymous reviewers for helpful suggestions. Financial support was provided by the Environmental Restoration Program at Los Alamos National Laboratory.
Literature Cited (1) Fisk, J.; Dempsey, C.; Fredricks, S.; Bottrell, D.; Williams, L. Presented at the EPA Conference on Field Screening Methods for Hazardous Waste Site Investigations, Las Vegas, NV, 1988. (2) Furst, G. A.; Tillinghast, V.; Spittler, T. Proceedings of the National Conference on Management of Uncontrolled Hazardous Waste Sites, Washington, DC, 1985. (3) Chappell, R. W.; Davis, O. A.; Olsen, R. L. Presented at the Screening Techniques and Analysis Conference, 1987.
(4) Leyden, D. E. Spectroscopy 1988, 2, 28. (5) Jenkins, R. A.; Dyer, F. F.; Moody, R. L.; Bayne, C. K.; Thompson, C. V. Army Report No. MIPR0389. Oak Ridge National Laboratory Report ORNL/TM-11385; 1993. (6) Watson, W.; Walsh, J. P.; Glynn, B. Am. Lab. 1989, 21, 60. (7) Walsh, J. P.; Harding, A.; Aulenbach, S. Presented at the Billings Symposium on Planning, Rehabilitation and Treatment of Disturbed Lands, Billings, MT, 1990. (8) Kuharic, C. A.; Cole, W. H.; Singh, A. K.; Gonzales, D. U.S. Environmental Protection Agency, U.S. Government Printing Office: Washington, DC, 1993; EPA-600/R-93/073. (9) de Boer, D. K. G.; Borstrok, J. J. M.; Leenaers, A. J. G.; van Sprang, H. A. X-Ray Spectrom. 1993, 22, 33. (10) Perlis, R.; Chapin, M. Presented at the EPA Conference on Field Screening Methods for Hazardous Waste Site Investigations, Las Vegas, NV, 1988. (11) Pella, P. A.; Tao, G. Y.; Lachance, G. X-Ray Spectrom. 1986, 15, 251. (12) Bilbrey, D. B.; Bogart, G. R.; Leyden, D. E.; Harding, A. R. X-Ray Spectrom. 1988, 17, 63. (13) Criss, J. W.; Birks, L. S. Anal. Chem. 1968, 40, 1080. (14) Lachance, G. R.; Traill, R. J. Can. Spectrosc. 1966, 11, 43. (15) Longmire, P.; Reneau, S.; Watt, P.; McFadden, L.; Gardner, J.; Duffy, C.; Ryti, R. Natural Background Geochemistry, Geomorphology, and Pedogenesis of Selected Soil Profiles and Bandelier Tuff, Los Alamos, New Mexico; Report LA-12913-MS; Los Alamos National Laboratory: Los Alamos, NM, 1995. (16) Kane, J. S.; Wilson, S. A.; Lipinski, J.; Butler, L. Am. Environ. Lab. Appl. Note 1993. (17) National Institute of Standards and Technology. Addendum to SRM Certificates 2709, 2710, and 2711; U.S. Government Printing Office: Washington, DC, 1993.
Received for review October 5, 1995. Revised manuscript received February 22, 1996. Accepted March 15, 1996.X ES950744Q X
Abstract published in Advance ACS Abstracts, May 1, 1996.
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